(1) Field of the Invention
The present invention relates to image forming apparatuses such as printers and copiers, that perform printing by thermally fixing images formed on a recording sheet, and in particular to technology of controlling a fixing temperature between pages, which is applicable to an image forming apparatus that saves power by determining a target fixing temperature for each page according to the characteristics of images on the page.
(2) Description of the Related Art
In an image forming apparatus such as a printer and a copier, printing is performed by forming toner images on a recording sheet based on image data and thermally fixing the toner images by passing the recording sheet through a fixing position on a heating roller. The fixing temperature required for thermally fixing the toner images varies depending on conditions such as the amount of toner adhering to the recording sheet, and the characteristics of the images formed on the recording sheet.
In order to avoid a fixing failure under any conditions, it would be possible to set the target fixing temperature, which is the temperature to be maintained at the surface of the heating roller during the thermal fixing onto a recording sheet, to be a temperature that ensures preferable fixing even under the printing conditions that require application of the largest amount of heat. However, such a setting is not favorable in terms of power saving, because an unnecessarily large amount of power will be consumed for fixing toner images on a page that does not require such a large amount of heat.
As technology of saving power consumption, Patent Literature 1 (Japanese Patent Application No. 10-039673), for example discloses technology of changing the target fixing temperature of the surface of the heating roller for each page, depending on the characteristics of the images to be thermally fixed on the page. This technology controls the fixing temperature so as to fit the characteristics of the images on the page, thereby reducing the power consumption in the thermal fixing.
Furthermore, Patent Literature 1 (Japanese Patent Application Publication No. 2013-076890) discloses technology of reducing waiting time that occurs when changing the target fixing temperature for each page, particularly when raising the target fixing temperature.
Specifically, Patent Literature 2 discloses an image forming apparatus that, when the target fixing temperature for the succeeding page to a given page is higher than the target fixing temperature for the given page by no less than a predefined value, raises the fixing temperature step-by-step from the target fixing temperature for the page that immediately precedes the given page during the sheet interval period (i.e. the period during which the interval between two successive pages passes though the fixing position on the heating roller), thereby raising the temperature of the surface of the heating roller to the target fixing temperature for the succeeding page before performing the thermal fixing for the succeeding page.
Consequently, this technology reduces the increment of the temperature during the sheet interval period even when the difference between the target fixing temperatures for pages is large, and reduces the waiting time that occurs when raising the fixing temperature, without extending the sheet interval period.
According to the technology of waiting time reduction disclosed in Patent Literature 2, it is possible to reduce the waiting time when there is a page that immediately precedes the given page and that has the same target fixing temperature as the given page. However, if there is no such a page (e.g. when the given page is the first page), it is impossible to reduce the increment of the temperature during the sheet interval period, or to reduce the waiting time.